<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

        .box{
            width: 400px;
            height: 400px;
            background-color: blueviolet;
            margin: 0 auto;
            box-shadow: 0px 10px 20px rgba(0, 0, 0, 0.5);
        } 
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>